Scribe Notes on -
MPLS and Traffic Engineering
by
Soumya Sakha Tripathy
Y3111046
Comparison of Circuit Switching & Packet Switching :
- In case of circuit switching , a dedicated connection is first
created between the two communicating agents. Where as in case of
circuit-switching, no such dedicated connection is created.
- Both can handle failure in the route.
- Support switching better supports interactive traffic.
- Bandwidth is better utilized in packet switching ,as it
uses statistical multiplexing.
- Circuit switching is limited to scaling , because a switch state
per connection is maintained here.
- Circuit switch has high percentage of utilization . So, it is
used in voice systems , telephones etc. Packet Switching is used in
systems where channel utilization is less , like in the Web.
- Packet switching uses destination based
forwarding technique . Here there is a tree like structure of
prefixes. And each router searches for the longest prefix matching by
looking-up the tree.
Multi
Protocol Label Switching :
** Over View :
- MPLS combines
the advantages of both Circuit Switching and Packet Switching.
- MPLS layer
works between layer 2 and layer 3 i.e. link layer and network
layer. ( It is below Network/IP layer as it operates
within an ISP ).
- Multiprotocol
Label Switching (MPLS) traffic engineering software enables an MPLS
backbone to replicate and expand upon the traffic engineering
capabilities of Layer 2 ATM and Frame Relay networks.
- Traffic
engineering is essential for service provider and Internet service
provider (ISP) backbones. Such backbones must support a high use of
transmission capacity, and the networks must be very resilient, so that
they can withstand link or node failures.
- MPLS traffic
engineering provides an integrated approach to traffic engineering.
With MPLS, traffic engineering capabilities are integrated into Layer
3, which optimizes the routing of IP traffic, given the constraints
imposed by backbone capacity and topology.
- MPLS traffic
engineering routes traffic flows across a network based on the
resources the traffic flow requires and the resources available in the
network.
- MPLS traffic
engineering employs "constraint-based routing," in which the path for a
traffic flow is the shortest path that meets the resource requirements
(constraints) of the traffic flow. In MPLS traffic engineering, the
flow has bandwidth requirements, media requirements, a priority versus
other flows, and so on.
- MPLS traffic
engineering gracefully recovers to link or node failures that change
the topology of the backbone by adapting to the new set of constraints.
** MPLS Basics:
- MPLS does label switching.
- Each IP flow is assigned a label.
- Then these filows are sent through a predefined path called Label
Switched Path ( LSP ).
- Each
router forwards the packets based on the label , not on Destination IP
address.
Here is a simple LSP :-
Here the packets are sent from the sourse router Ingress
to the Destination router Egress. Each router in LSP adds the MPLS
header to the packets and forwards it . Note that the forwarding to the
next route in LSP is based on label not on IP Address.
** MPLS Header :
Label ( 20 bits
)
|
CoS ( 3 ) |
S |
TTL ( 8 bits)
|
L2 Header
|
MPLS Header
|
L3 Header
|
L3
Data
|
L2 Header
|
MPLS Header
|
MPLS Header
|
L3 Header
|
L3 Data
|
- MPLS header is 32 bit header.
- 20 bit label specifies label of flow.
- Cos : Class of Service.
- TTL : Time To Live i.e. How much time the packet is allowed
to live , before get discarded.
- The rest of the fields of the header are
self-explanatory.
**
Fault Tolerance :
- As told earlier , MPLS is robust to link failure i.e. it recover
from the failure and adopt some other topology.
- For example , in the above case, let the link marked as "X"
fails.
- Then MPLS choose some different path . Here are 2 choices.
- The first one is , it chooses another path between the routers
whose link is damaged . In the figure it is shown by YELLOW dotted line.
- The second option is to set a new path between routers Ingress
and Egress ( Shown by PINK dotted lines).
- The first option is called Local Recovery
where as the later one is End-to-End
Recovery.
References :